This application incorporates by reference of Taiwan application Ser. No. 090108205, filed on Apr. 4, 2001.
1. Field of the Invention
This invention relates in general to a method for compensating luminance, and more particularly to a method for compensating luminance, and the method is suitable for use in a plasma display panel.
2. Description of Related Art
With the rapid development of the fabrication technology of the audio/video (A/V) device, it can be foreseen that people in the future will enjoy the audio and video service with much higher performance than now. Taking the displaying device as an example, the conventional cathode ray tube (CRT) displaying device has not only the disadvantages of large volume and the serious radiation issue, but also that the image at the brim region of the screen has very serious image contortion and distortion problem. Therefore, the conventional CRT displaying device certainly cannot satisfy the people who desire to enjoy the audio and video service with higher performance. When the high definition digital television (HDTV) system starts to operate and broadcast in the near future, the conventional CRT displaying device designed with the analog manner will be gradually thrown away. Instead, the plasma display panel (PDP), which has at least advantages of low radiation, low power consumption, and large displaying area but small volume, can be a very promising product to replace the CRT displaying device.
Referring to FIG. 1, it is a drawing, schematically illustrating a loading effect on a plasma display panel. The loading effect is a phenomenon that the luminance of a pixel is affected by the luminance of the other pixels which are in the same row with the pixel. The loading effect causes a difference between the actual luminance and the expected luminance of the pixel and the actual luminance is less than the expected luminance. As shown in FIG. 1, the panel includes three pixel rows 102, 104, and 105. The pixel row 102 includes pixel sub-rows 106 and 110, the pixel row 104 includes pixel sub-rows 108 and 112, and the pixel row 105 includes pixel sub-rows 109 and 113. The luminance of the pixel sub-row 106 is less than the pixel sub-row 108 and the luminance of the pixel sub-row 108 is less than the luminance of the pixel sub-row 109. In addition, each pixel of the sub-rows 110, 112, and 113 has the same pixel signal. Each pixel signal includes a luminance value to determine the luminance of the pixel. In theory, the luminance of pixel sub-rows 110, 112, and 113 should be the same. However, in practical, the pixel sub-rows 110, 112, and 113 have the individual luminance L110, L112, and L113, respectively. The relation among L110, L112, and L113 is L110 greater than L112 greater than L113. And the luminance of pixel sub-rows 110, 112, and 113 are all less than the expected luminance which is corresponded to the luminance value of the pixel signals. In other words, the luminance of the pixel is affected by the luminance of the other pixels in the same row which causes the actual luminance is less than the expected luminance of the pixel. If the luminance of the other pixels in the same row is larger, the difference between the actual luminance and the expected luminance of the pixel will be larger. This phenomenon is called the loading effect.
A frame data is composed of a number of pixel signal rows. A plasma display panel with a resolution of 800xc3x97600 is taken as an example, in which it means that the whole panel has 800 pixels for each row, and 600 pixels for each column. The frame data should include 600 pixel signal rows. Each pixel signal row has 800 pixel signals. Each pixel signal includes a luminance value. The luminance value of the pixel signal is represented by a eight-bit binary number. The range of the luminance value is from (00000000)2 to (11111111)2 represented by binary numbers or from 0 to 255 represented by decimal numbers. The larger the luminance value of the pixel signal is, the larger the luminance of the pixel which receives the pixel signal will be. The smallest value 0 corresponds to the full black luminance of the pixel and the largest value 255 corresponds to the full white luminance of the pixel respectively.
Referring to FIG. 2, it is a block diagram, schematically illustrating a conventional device for compensating luminance 200. The conventional device for compensating luminance 200 includes a field programmable gate array (FPGA) 202 and two memory devices 204 which are coupled to the FPGA 202 respectively. The conventional device for compensating luminance 200 can determine whether or not the luminance compensation is necessary to be performed. If performing luminance compensation is determined to be necessary, the device for compensating luminance 200 will perform luminance compensation on the pixel signal row.
Referring to FIG. 3, it is a flow chart, schematically illustrating the conventional luminance compensation method performed by the device for compensating luminance. The conventional method for compensating luminance of the plasma display panel is described in the following. First, in the step 302, the device for compensating luminance will sequentially read each pixel signal of the pixel signal row.
Then, in the following step 304, the device for compensating luminance computes the load of the pixel signal row. According to forgoing descriptions, if the luminance of the other pixels in the same row is larger, the actual luminance of the pixel will be less and the difference between the actual luminance and the expected luminance will be larger. Therefore, the degree of the loading effect is determined by the luminance of the pixel row. When the device for compensating luminance determines whether or not luminance compensation is necessary to be performed, the magnitude of the luminance value of each pixel signal in the pixel signal row is a factor for consideration. In the conventional method for compensating luminance, the load of the pixel signal row is defined as the number of pixel signals in the pixel signal row that the luminance value of the pixel signals is larger than the predetermined threshold luminance value. The magnitude of the load of the pixel signal row determines the degree of the loading effect of the pixel row in the plasma display panel. The device for compensating luminance computes the load of the pixel signal row by comparing luminance value of each pixel signal to the threshold luminance value. According to the forgoing descriptions, each luminance value is represented by a binary number of eight bits. The higher bit has more influence than the lower bit of the luminance value on the determination of the luminance of the pixel. Therefore, when comparing luminance value of each pixel signal with the threshold luminance value, only the highest-three-bit of the pixel signal is needed to be put into consideration. The highest-three-bit of the threshold luminance value is defined to be 101 in conventional luminance compensation method. If the highest-three-bit of the luminance value is larger than or equal to 101, the luminance value of this pixel signal is considered as the one to increase the degree of the loading effect. On the contrary, if the highest-three-bit of the luminance value is less than 101, the luminance value of this pixel signal is considered as the one not to increase the degree of the loading effect. The load of the pixel signal row is the number of the pixel signal in the pixel signal row that the luminance value of the pixel signals is larger than or equal to (10100000)2.
Then, in the following step 306. If the load of the pixel signal row is larger than or equal to the predetermined threshold load, luminance compensation will be performed. If the load of the pixel signal row is less than the threshold load, luminance compensation will not be performed and the pixel signal row will be outputted directly form the device for compensating luminance.
If the load of the pixel signal row is larger than or equal to the threshold load, luminance compensation will be performed. In the step 308, the luminance compensation value is decided according to the load of the pixel signal row. Each load of the pixel signal row that the magnitude of the load is larger than the threshold load is corresponded to a luminance compensation value. A number of luminance compensation values are predetermined and stored in the form of look-up-table (LUT) in the device for compensating luminance. The proper luminance compensation value for the pixel signal row is decided by looking up the look-up-table for luminance compensation. All luminance compensation values in the look-up-table are arranged in a decreasing order according to the magnitude of the luminance compensation values. When performing luminance compensation, the device for compensating luminance will choose the luminance compensation value from the top of the look-up-table. The first luminance compensation value is the largest of all luminance compensation values in the look-up-table. After then, in the step 310, the luminance compensation value is added to the luminance value of each pixel signal in the pixel signal row so as to perform luminance compensation. The pixel signal row that the luminance value of each pixel signal is added with the luminance compensation value is defined as a compensated pixel signal row.
After performing luminance compensation, the magnitude of the luminance value of each pixel signal in the pixel signal row will be larger than the original luminance value and the pixel row which displays according to the compensated pixel signal row will be brighter than the pixel row which displays according to the original pixel signal row. Therefore, the purpose of compensating luminance can be achieved.
Then, in the following step 312, the device for compensating luminance will compute the load of the compensated pixel signal row. Afterwards, in the following step 314, the load of the compensated pixel signal row is compared with the load of the original pixel signal row to see whether they are the same or not. If the magnitude of the luminance compensation value is too large, the pixel row which displays according to the compensated pixel signal row will be much brighter than the other pixel row in the panel and It will affects the displaying performance of the panel. Therefore, the device for compensating luminance will compare the load of the compensated pixel signal row with the load of the original pixel signal row to see whether they are the same or not. If the magnitude of the load of the compensated pixel signal row and the original pixel signal row are the same, it is considered that the magnitude of the luminance compensation value is proper. If the load of the compensated pixel signal row is larger than the load of the original pixel signal row, it is considered that the magnitude of the luminance compensation value is too large and the device for compensating luminance will repeat the step 308, the next luminance compensation value is chosen from the look-up-table. Since the luminance compensation values in the look-up-table are arranged in decreasing order, the magnitude of the newly chosen luminance compensation value is less than the previously chosen luminance compensation value. Then, the step 310, 312, and 314 are repeated again until the load of the compensated pixel signal row is the same with the load of the original pixel signal row. It is considered that the magnitude of the luminance compensation value is proper and the displaying performance of the panel cannot be affected. At last, the step 316 is performed, in which the compensated pixel signal row is outputted from the device for compensating luminance and the conventional luminance compensation method is accomplished.
There are some disadvantages of the conventional method for compensating luminance. First, in conventional luminance compensation method, the threshold luminance value and the threshold load used to determine the load of the pixel signal row are fixed. And the magnitude and the order of all luminance compensation values in the look-up-table are also fixed. When performing luminance compensation, no matter how large the load of the pixel signal row is, the same look-up-table is used to decide the luminance compensation value. It is not taken into consideration that luminance compensation values of the look-up-table must be adjusted according to the magnitude of the pixel signal to achieve a better effect of luminance compensation.
Second, when luminance compensation is performed, the luminance value of each pixel signal belonging to the same pixel signal row is added with a luminance compensation value regardless of the magnitude of each pixel signal. Even though the smallest pixel signal 0 which cannot increase the degree of the loading effect is still added with the luminance compensation value. Therefore, the luminance of the whole panel will become larger after luminance compensation is performed. It affects the displaying performance of the panel because the full dark luminance cannot be displayed and the luminance contrast of the panel is reduced.
Third, the conventional method for compensating luminance is to directly add the luminance compensation value into the pixel signal. If the magnitude of the pixel signal in the pixel signal row is larger, the degree of the loading effect will become larger and the magnitude of the luminance compensation value should be larger to compensate the decreasing luminance of the pixel. However, since the magnitude of the pixel signal in the pixel signal row is large, it will be more difficult to add the large magnitude luminance compensation value into the pixel signal and the purpose of luminance compensation will be more difficult to achieve. For example, if there is one pixel signal which has the largest luminance value 255, luminance compensation cannot be performed on the pixel signal row no matter how large the load of the pixel signal row is.
Fourth, if there is one pixel signal that the luminance value of the pixel signal is (10011111)2, the load of the compensated pixel signal row is always larger than the load of the original pixel signal row when performing luminance compensation. In other words, if there is one pixel signal that the luminance value of the pixel signal is (10011111)2, luminance compensation will not able to be performed.
According to the foregoing descriptions, the conventional method for compensating luminance has several disadvantages as follows: first, the magnitude of the luminance compensation value cannot be adjusted according to the load of the pixel signal row. Second, the full dark luminance cannot be displayed and the luminance contrast of the panel is reduced. Third, if the load of the pixel signal row is larger, it is more difficult to perform luminance compensation. Fourth, if there is one pixel signal that the luminance value of the pixel signal is (10011111)2, luminance compensation cannot be performed.
It is therefore an objective of the present invention to provide a method for compensating luminance, suitable for use in a plasma display panel, so as to achieve the following objectives: First, The magnitude of luminance compensation signal can be adjusted according to the load of the pixel signal row. Second, the full dark luminance can be displayed and the luminance contrast of the panel is not reduced after performing luminance compensation. Third, it will not occur that luminance compensation cannot be performed due to some specific situation.
According to the objectives of the present invention, it is provided a method for compensating luminance, suitable for use in a plasma display panel. The plasma display panel includes a device for compensating luminance to perform luminance compensation on the pixel signal row. The method for compensating luminance includes the following steps. First, each pixel signal in the pixel signal row is read out and stored. Then, the load of the pixel signal row is computed. The load is defined as the number of pixel signal in the pixel signal row that the luminance value of the pixel signal is larger than a predetermined threshold luminance value. Then, a primary luminance compensation value is decided according to the load of the pixel signal row. After that, a number of secondary luminance compensation signals are decided according to the primary luminance compensation value. Afterwards, luminance compensation is performed. If the luminance value of the pixel signal is larger than or equal to a predetermined low threshold luminance value, the pixel signal is subtracted by the primary luminance compensation value. Otherwise, the luminance value of the pixel signal is subtracted by a corresponding secondary luminance compensation value according to the magnitude of the luminance value. At last, the compensated pixel signal row is outputted.